Wrap the bulk of kvm_cpu_exec with cpu_exec_start/end, so that kvm
version can also enjoy performing certain operations while all vCPUs are
quiescent.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20170606181948.16238-15-rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>